Due to the sense of acceleration, McKee argues in Story that the last act of the film should be the shortest: possibly 20 minutes or less of a two-hour film. (219) He describes the third act climax is the obligatory scene set up in the inciting incident.
As Mamet notes in the final chapter of Three Uses of the Knife, the third act has unique complications for the writer, which become the audience’s problems by default. The tired writer knows the fate of the equally tired protagonist, and all that’s left is uniting the previously introduced elements for the finale. The thesis of the first act and the antithesis in the second act achieve a form of synthesis. The jealous husband remembers that a knife can do more than cut his bread and shave his beard. Johnny Depp loses the big race with honor, but wins Cate Blanchett’s heart. The truth prevails and all is made whole, a ritual which enables “cleansing awe,” which Mamet suggests is similar to religious confession.
Often this is what defines the film. It is the silhouette you remember years after you’ve left the theater. It’s the struggle between JJ Gites and Noah Cross in Chinatown after Eveyln Mulwray’s secret has been revealed, or King Kong in Manhattan.
In many cases, the third act comes with a change in setting. In Hellboy 2, when the heroes arrive at the resting place of the golden army, the third act has become. Sometimes, it’s the final battle. So in the Avengers, it’s the entire Manhattan under siege sequence when the alien army finally shows up. In The Amazing Spider-Man, it’s Spider-Man’s quest to get to Oscorp in time to stop the Lizard, even if the cops want to kill him.